A switching regulator for generating a direct current by switching a direct current at a high frequency, transforming the resultant current in a transformer and then rectifying the transformed current has advantageous features that it is constructed in small dimensions and operates at a high efficiency. By virtue of the foregoing advantageous features, the switching regulator has been widely used as a power supply unit for various electric instruments and equipments. In recent years, to reduce a load to be borne by a switching element and moreover improve operational efficiency, a current resonance converter of the type including a switching regulator added with a resonant circuit has been used increasingly.
With respect to the current resonance converter as constructed in the above-described manner, it is important from the viewpoint of protection of a switching element and an operational efficiency that an ON/Off operation is performed by the switching element while an electric current is maintained in the zero state (zero-current switching). To perform a zero-current switching operation, it is required that an intensity of electric current flowing through a resonance circuit assumes a value lower than a predetermined one. As the intensity of the electric current flowing through the resonant circuit is increased, components constituting the resonant circuit are excessively heated, resulting in the running life of each component being shortened. Under the foregoing circumstances, the current resonance converter is required to include a protection circuit for protecting it from the influence of an overcurrent.
To protect the current resonance converter from the influence of an overcurrent, a current transformer ha been hitherto arranged on the primary coil side of a transformer to measure an alternate current. Driving of the switching element is controlled based on the direct current which has been generated by detecting the alternate current, rectifying the alternate current and then smoothing the resultant current.
However, as long as the alternate current is detected by using the current transformer, it is practically difficult to protect the current resonant converter from an abnormal state such as short circuit or the like malfunction of an inductor and a capacitor constituting the resonant circuit. Another problem is that the conventional current resonance converter has a degraded property of responsiveness in respect of protection of the switching element because of arrangement of a smoothing circuit for smoothing an electric current outputted from the current transformer. Another problem is that the conventional current resonance converter is constructed in larger dimensions.
As described above, with respect to the protection circuit for the conventional current resonant converter, since an electric current flowing through a primary coil of the transformer is detected by the current transformer, it is difficult to detect an abnormal state due to short circuit of an inductance and a capacitor constituting a series resonant circuit. Consequently, the conventional current resonance converter has a degraded property of responsiveness in respect of protection of the switching element. In addition, the conventional current converter is unavoidably constructed in large dimensions with a heavy weight because of arrangement of the current transformer.
The present invention has been made in consideration of the aforementioned problems to be solved.
An object of the present invention is to provide a current resonance converter which makes it possible to protect the converter from an abnormal state due to short circuit or the like malfunction of an inductor and a capacitor constituting a resonant circuit.
Another object of the present invention is to provide a current resonance converter which has an excellent property of responsiveness in respect of protection of a switching element and moreover assures that the converter can be constructed in smaller dimensions with a lighter weight.